When does the 90-day period for a non-O visa start: upon grant or entry into Thailand?

You have 90 days to use your visa then 90 days to stay once you get there would be a simple explanation

Both, as above

There are 2 dates. The first date is the visa validity date. The visa will be valid for 90 days from the day it is issued by the embassy. The expiration date will be on the visa itself.

You must enter Thailand by this date to use your visa.

Once you enter visa, you'll get the number of days granted by your visa, which for non-O would be 90 days. This 90 days will start the day you enter Thailand.